1. Field of the Invention
The present invention relates to imaging systems, and in particular, to methods and systems for color compensation.
2. Description of the Related Art
Conventional integrated circuit imaging devices include an array of light detecting elements or pixels which are interconnected to generate a signal representation of an image illuminating the device. Two common examples of conventional integrated circuit imaging devices are a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensing device. Conventional imaging devices use one or more light detecting elements and charge storage elements. In order to produce a color image, the imaging devices separate the light into various color components by filtering the light before the light strikes the light detecting elements. The array of light detecting elements is often deposited with a filter layer such that neighboring pixels may have different color filters and organized in a particular pattern.
Because each pixel is only capable of detecting a single color, conventional imaging devices require a process by which all of the color components are reconstructed for each pixel in order to maintain the original unfiltered array resolution. To reconstruct the color components, conventional imaging devices use a process of color interpolation that is performed after an analog signal associated with each pixel has been digitized. The conventional process of color interpolation performed after an analog signal associated with each pixel has been digitized requires conversion from analog to digital (A/D) and may require extensive computations in order to achieve a high quality color presentation of the image. The A/D conversion and extensive computations may require hardware, such as analog-to-digital (A/D) converters, memory, processors and software. The hardware and software may add to the complexity, size and expense of the imaging device and reduce the speed of the imaging process.
Conventional imaging devices also require color compensation for differences in the response of the various color filters and for variations within the integrated circuit sensor array, such as process, materials, temperature or manufacturing. For example, when the primary color scheme is used, the response of an element that absorbs red light may be different than an element that absorbs blue light even when illuminated by light of equal red and blue luminosity levels. When exposed to a flat light image having equal intensity and chromatisity levels throughout, typical CMOS or CCD arrays may generate analog signals having significant magnitude variations for the different color components. Accordingly, if the analog signals are used to reproduce the original image, the reproduced image colors will not match the original colors.
To overcome this problem, conventional imaging devices employ a process of color correction that is performed after the analog signals for each pixel have been digitized. One drawback of the conventional color correction process is a loss in color dynamic range that results from under-utilization of the A/D converter for some of the color components. Another drawback is increased computations that translates into additional hardware, size, expense and/or reduced speed.
The effect of loss in color dynamic range is particularly noticeable in the low light areas of an image that contains low as well as high light regions. The human eye is sensitive to minute changes in hue and saturation levels. A reduction in color component dynamic range may result in less vivid, plain, or flat images. Attempts to correct this via hue or saturation enhancement filters may cause color distortion rather than color restoration.
Another color compensation that integrated circuit color imaging devices require is for different illumination temperatures. The hue of a color component changes with respect to the ambient illumination. Thus, a white object under sunlight conditions is perceived by the imaging device as white, but under fluorescent light conditions is perceived as light green.
To overcome this problem, conventional integrated circuit color imaging devices employ a process of white balance that is performed after the analog signals for each pixel have been digitized. Typically, each of the three processed color components, red, green or blue, is a convolution of the three colors. A drawback of the conventional white balance process is increased computations that translates into additional hardware and/or reduced speed.
The collection of signals read from the pixels represents the image viewed by the array. Each pixel represents a sample of the image and hence is a data value in the two-dimension image produced by the imaging system. Defect pixels, referred to as ‘bad pixels,’ do not contain a correct value and appear as artifacts. The bad pixels can reduce the image quality significantly. A bad pixel is caused by array defect and produces an output signal that significantly deviates from the mean output level of adjacent pixels when the exposure level of all pixels is substantially unified. Pixels that are significantly brighter than adjacent pixels in a unified dark frame are referred to as ‘hot pixels,’ while pixels that are significantly darker than adjacent pixels in a unified bright frame are referred to as ‘dead pixels.’
The defect pixels are typically distributed in a random manner. However, a bad column (i.e.—a complete column is defective), or a blemish (i.e.—a cluster of neighboring pixels is defective) may occur and are typically discarded by the manufacturer. The manufacturer releases sensor arrays that contain random defect pixels in an amount that does not exceed a given limit, and the bad pixels are typically corrected. Both CCD and CMOS integrated circuit color imaging devices employ a process of bad pixel detection and correction. Conventionally, the detection step is performed off-line by the manufacturer. A bad pixel list is stored in the device. The correction step is typically performed after the analog signal for each pixel has been digitized.